In primary cementing operations carried out in oil and gas wells, a hydraulic cement composition is disposed between the walls of the wellbore and the exterior of a pipe string, such as a casing string, that is positioned within the wellbore. The cement composition is permitted to set in the annulus thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the wellbore and bonds the pipe to the walls of the wellbore whereby the undesirable migration of fluids between zones or formations penetrated by the wellbore is prevented.
One method of primary cementing involves pumping the cement composition down through the casing and then up through the annulus. In this method, the volume of cement required to fill the annulus must be calculated. Once the calculated volume of cement has been pumped into the casing, a cement plug is placed in the casing. A displacement fluid (e.g. drilling mud) is then pumped behind the cement plug such that the cement is forced into and up the annulus from the far end of the casing string to the surface or other desired depth. When the cement plug reaches a float shoe disposed proximate the far end of the casing, the cement should have filled the pre-designed or entire volume of the annulus. At this point, the cement is allowed to dry in the annulus into the hard, substantially impermeable mass.
As the drilling industry continues to shift towards harsher environments of high pressure and high temperature as a result of ultra-deepwater wells, mature fields, and unconventionals, formation's pore pressure and fracture gradient margins are becoming narrower. As a result, it has been found that due to the high pressure at which the cement must be pumped, at a pressure above the hydrostatic pressure of the cement column in the annulus plus the friction pressure of the system (ECD, Equivalent Circulating Density=Phydrostatic+Pfriction), fluid from the cement composition may leak off into a low pressure zone traversed by the wellbore, especially where the pore pressure/fracture gradient margins are very low. When such leak off occurs, the remainder of the cement composition near this low pressure zone is not sufficient to provide optimum zonal isolation to the required zone. Thereafter, remedial cementing operations, commonly referred to as squeeze cementing, must be used to place cement in the remainder of the annulus.
Accordingly, prior art attempts have been made to avoid the problems associated with fluid leak off into low pressure zones during cementing operations, especially for narrow margin cases. One method of avoiding such problems is called reverse cementing wherein the cement composition is pumped directly into the annulus. Using this approach, the pressure required to pump the cement to the far end of the annulus is much lower than that required in conventional cementing operations. Thus, significantly reducing the cement pumping pressure, and therefore the ECD, which in turns, diminishes the likelihood of fracturing the formation and having significant losses before the entire annulus or intended zone is filled with cement is significantly reduced.
It has been found, however, that with reverse cementing it is necessary to identify when the cement begins to enter the far end of the casing and reaches the desired depth inside the casing to leave the desired shoe track length such that the cement pumps may be shut off. Continuing to pump cement into the annulus after cement has reached the desired location after having crossed the far end of the casing, forces undesired amounts of cement into the casing, which in turn may necessitate additional drill out times.
One method of identifying when the cement has reached the far end of the annulus involves running a neutron density tool down the casing on an electric line. The neutron density tool monitors the density out to a predetermined depth into the formation. When the cement begins to replace the drilling mud in the annulus adjacent to the neutron density tool, the neutron density tool senses the change in density and reports to the surface that it is time to stop pumping additional cement into the annulus. Another method of identifying when the cement has reached the far end of the annulus involves running a resistivity tool and a wireless telemetry system down the casing on a wireline. The resistivity tool monitors the resistivity of the fluid in the casing such that when the cement begins to replace the drilling mud in the casing, a wireless signal is sent to the surface indicating it is time to stop pumping additional cement into the annulus.
It has been found, however, that use of such retrievable tool systems is prohibitively expensive. In fact, numerous neutron density tools and resistivity tools have been ruined during such operations as a result of the cement entering the far end of the casing and contacting these tools.
Therefore, a need has arisen for a system and method for cementing the annulus between the wellbore and the casing that does not require pumping the cement at pressures that allow for leak off into low pressure zones, especially for narrow margins operations. A need has also arisen for such a system and method that identify when to stop pumping additional cement into the wellbore. Further, a need has arisen for such a system and method that do not require the use of expensive equipment including tools that must be retrieved from the well once the cementing operation is complete.